How to add a gadget/widget inside your Blogger Blog header

Maybe you have seen that many blogs have AdSense Units or other ads and
widgets in the header section of the blog, the most popular format being
the AdSense 468x60 ad unit - see in the screenshot below:

But how we can add something inside blogger header - be it a Adsense
unit, a search bar, social media buttons or a widget? By simply adding a
section in the header. First off, to give space for the extra gadget to
fit in, our blogger header should be resized.